1. Field of the Invention
The present invention relates to a solid-state image sensor and, more particularly, to a means for controlling blooming in an image-sensing element of such a sensor.
2. Description of the Prior Art
Solid-state image sensors are known in which signal carriers are generated in a photosensitive element in proportion to the luminous energy directed on the element. The signal carriers are read out by means of a charge-coupled device (CCD). When the quantity of light incident on the photosensitive element for a given period of time is relatively high, carriers are produced in the photosensitive element in excess of the storage capacity of the element; when this happens, the excess carriers flow into adjacent photosensitive elements. This results in a phenomenon known as blooming. In the reproduction of an image from signals generated by the sensors, the image quality is greatly degraded in areas where blooming occurred when the image was captured.
One known method of controlling blooming is through the use of vertical-overflow drains for removing excess carriers. In one such device, an N-type photosensitive element is separated from an N-type substrate by a P-type layer which is at a reference potential. When the substrate is biased above a predetermined potential, a punch through condition is obtained in which the P-type layer is in depletion condition. When this occurs, excess carriers from the photosensitive element can flow into the substrate which serves as the vertical overflow drain. A disadvantage of sensors which use vertical overflow drains is that the sensors exhibit a loss in responsivity in the green and red regions of the spectrum.
It is also known to use so-called lateral-overflow drains to control blooming in image sensors. In U.S. Pat. No. 4,460,912, there is disclosed a solid-state sensor of the interline transfer type in which photosensitive elements are arranged in vertical columns with a vertical transfer portion provided at one side of each of the photosensitive elements. On an opposite side of the photosensitive elements, a lateral-overflow drain is formed opposite each of the elements. An overflow gate control electrode is provided between each photosensitive element and its corresponding overflow drain, and a D.C. voltage is applied to the electrode to control the transfer of excess carriers to the overflow drain. A problem with this device is that there is a loss in the effective area of the sensor because of the additional control electrode for the overflow gate. Another problem is that it is difficult to manufacture the device with a pixel as small as desired because of the relatively large number of surface elements.